Field of the Invention
The present invention relates to a backdrilling method and a backdrilling apparatus for a multilayer printed wiring board.
Description of the Related Art
In order to mount electronic components, such as large-scale integrated circuits (LSIs), on multilayer printed wiring boards, through-holes are formed as terminals to connect to predetermined internal wiring layers, and the through-holes are plated with conductive materials. However, because the plated portions in the through-holes are longer than desired, problems may be caused in some cases, such as impedance mismatch, signal delay or rounded waveform, unless the excessively long portions (hereinafter referred to as “stubs”) are shortened.
Thus, backdrilling is required to remove such excessively plated portion, which is a stub, using a drill bit with a slightly larger size than the through-hole size from the backside. In backdrilling, it is important how the stub is removed. In this regard, one problem is how to control the depth of backdrilling.
Conventionally, in order to accurately control the drilling depth, an internal conductor layer for detecting a reference depth (hereinafter referred to as a “reference depth detection layer”) is formed in advance near a backdrilling portion in a multilayer printed wiring board, as disclosed in JP-A-2009-4585. In this technique, drilling is performed toward the reference depth detection layer prior to backdrilling so as to measure the reference depth, and then the depth control of backdrilling is performed based on the reference depth.
In this conventional technique, it is necessary to allocate a region for the reference depth detection layer (hereinafter referred to as a “reference depth detection region”) as close to the backdrilling portion as possible in the multilayer printed wiring board. However, because the reference depth detection region is useless for the formation of the intended circuit in other words, it is desirable to allocate, if possible, the reference depth detection region in the outer marginal region of the board away from the backdrilling portion.
However, a multilayer printed wiring board is typically formed by heating and compressing a alternate stacking of resin layers and conductive wiring layers. Thus, as illustrated in FIG. 4, the central portion of a board may become thicker than the outer marginal region, and the depth of the internal wiring layer may vary as the board thickness varies. FIG. 4 is a cross-sectional view illustrating the state of such variation in an exaggerated manner.
Thus, when the reference depth detection region is allocated apart from the circuit formation region in the central portion of the board, for example, the reference depth of the internal wiring layer obtained in the reference depth detection region may become useless to determine the depth in the central portion. As a result, it may become impossible to ensure the depth accuracy for backdrilling in the circuit formation region in the central portion of the board.
JP-A-H3-3009 describes a technique for ensuring the depth accuracy by measuring the board thickness at a machining position and then calculating the drilling depth by multiplying the board thickness by a predetermined ratio, even when the thicknesses of the layers of the multilayer printed wiring board or its board thickness vary.
However, JP-A-H3-3009 does not describe how and when the predetermined ratio is obtained. Specifically, it does not clarify the relationship between the method for calculating the ratio and the method for using the ratio, particularly in regards to backdrilling.